As you most likely know validates_uniqness_of :foreign_id does not allow nil values by default.

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To allow nil one has to set the :allow_nil => true option.

Very unexpected scoping this validation will not raise an error if foreign_id set to nil for the first created record of this kind.

validates_uniqueness_of :foreign_id, :scope => :another_column # allows foreign_id to be nil

Without a validation for presence of foreign_id now unusual records could be created.